Because of the erosive and corrosive character of the constituents that they contain, the combustion products of certain "dirty" fuels, notably coal, are often unsuitable for use directly as the working fluid for gas turbines and the like. It is of course common practice to utilize heat exchangers to convectively transfer thermal energy from a hot, dirty gas to a heat-transfer fluid, thereby reducing the deleterious effects of the gas by subjecting only stationary parts to it. Further protection can be afforded by sweeping the exposed surfaces of the heat exchanger with a relatively clean (or only moderately dirty) gas; it is believed that proposals have been made to aerodynamically clean high temperature heat exchangers for use in magnetohydrodynamic recuporators. (Hoover et al; NASA Final Report No. NAS-3-19407, 1976)
As far as is known, no method or apparatus has heretofore been provided by which the energy produced by the burning of coal, char, and other dirty fuels, can be recovered in a highly efficient and yet practical manner while, at the same time, effectively shielding the energy-recovery structures from erosive and corrosive components of the combustion gases.